Field of the Invention
The present invention relates to an image pickup apparatus, or more specifically, to an image pickup apparatus which implements countermeasures for smear phenomena of the image pickup apparatus, and realizes an increase in sensitivity of the image pickup apparatus.
Description of the Related Art
Solid-state image pickup elements have heretofore been used in image pickup apparatuses, and CCD sensors and CMOS sensors have been widely used as such solid-state image pickup elements. There has been known a smear phenomenon, in which a bright stripe noise appears in a column direction (a vertical direction) of the image pickup elements in the case of using the CCD sensors. When there is a high-brightness photographic subject on a picture plane, for example, the smear phenomenon in the CCD sensors occurs due to a situation where undesired charges are accumulated in the image pickup elements by way of exposure during vertical transfer and are mixed into signal charges being transferred on a vertical transfer path. Since such smear is attributed to the structural problem of the CCDs, the case of using the CMOS sensors does not cause the smear in the column direction in shooting a high-brightness photographic subject. The latter case, however, may cause a lateral smear phenomenon in which a level variation appears in the form of a horizontal line or a horizontal band in a row direction (a lateral direction) in a region where a high-brightness photographic subject is present.
Such a smear phenomenon in the CMOS sensors is thought to be attributed to wiring layouts for an electric source, ground, and so forth. When the high-brightness photographic subject forms an image at a particular portion on the picture plane, a large amount of charges are generated in such a high-brightness photographic subject region. In this case, outputs from a pixel unit, a vertical output line, a column amplifier unit, and the like may vary significantly and cause a variation in an electric source shared by the components on the same row. As a consequence, the level variation occurs across the same row, or in other words, the lateral smear phenomenon comes into being.
According to Japanese Patent Application Laid-Open Nos. 2010-093389 and 2014-165676, an output signal of an effective pixel part of an image pickup element is corrected on the basis of an output signal of a light shielding pixel part provided in a region outside of the effective pixel part. These publications state that it is thus possible to reduce smear phenomena that occur in image pickup apparatuses adopting the CCD sensors and the CMOS sensors.
According to Japanese Patent Application Laid-Open No. 2013-085110, in an electric source line for a sensor column circuit, a gate voltage of a current source transistor operating in a saturation region relative to an electric source voltage is sampled and stored. By controlling such that the gate voltage becomes equal to the sampled voltage, the variation in electric source voltage is suppressed and the smear phenomenon is suppressed as a consequence.
The above-described conventional techniques have a difficulty in dealing with various shooting scenes at day and night. For example, according to Japanese Patent Application Laid-Open Nos. 2010-093389 and 2014-165676, there may be a case where the effect to suppress the smear phenomenon can be somewhat produced but is still insufficient for full correction, which is likely to cause deterioration in image quality.
While the technique according to Japanese Patent Application Laid-Open No. 2013-085110 can curb the smear phenomenon by suppressing the variation in electric source voltage, each image pickup element is saturated with a smaller number of electrons than the saturation number of electrons intrinsic to the image pickup element. In other words, this technique has a problem of reduction in dynamic range of image pickup signals obtained.
Meanwhile, when a gain is increased for shooting at night in order to increase sensitivity, there is a risk of amplifying a smear component that fails to be properly corrected, thereby causing further deterioration in image quality and bringing about an ugly shot image. In this regard, it is a general practice to set a gain upper limit in advance.